The Internet Protocol (IP) multimedia subsystem (IMS), defined by the 3rd Generation Partnership Project (3GPP), is an architectural framework for implementing IP-based telephony and multimedia services. IMS defines a set of specifications that enables the convergence of voice, video, data and mobile technology over an all IP-based network infrastructure. In particular, IMS fills the gap between the two most successful communication paradigms—cellular and Internet technology, by providing Internet services everywhere using cellular technology in a more efficient way. Session Initiation Protocol (SIP) is the main protocol for IMS. SIP is an application layer control (signaling) protocol for creating, modifying and terminating sessions with one or more participants.
Various proprietary formats, outside of standard SIP, currently exist in SIP. Examples of such SIP formats include SIP-T (SIP for Telephones), SIP-LMSD (SIP-Legacy Mobile Station Domain), and SIP-I (SIP with encapsulated ISUP (Integrated Services Digital Network User Part)).